AD9251BCPZRL7 Analog Devices, Inc., AD9251BCPZRL7 Datasheet - Page 21

• Manufacturer Part Number: AD9251BCPZRL7
• Description: 14-bit, 20 Msps/40 Msps/65 Msps/80 Msps, 1.8 V Dual Analog-to-digital Converter
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
VOLTAGE REFERENCE
A stable and accurate voltage reference is built into the AD9251.
The input range can be adjusted by varying the reference voltage
applied to the AD9251, using either the internal reference or an
externally applied reference voltage. The input span of the ADC
tracks reference voltage changes linearly. The various reference
modes are summarized in the sections that follow. The Reference
Decoupling section describes the best practices PCB layout of
the reference.
Internal Reference Connection
A comparator within the AD9251 detects the potential at the
SENSE pin and configures the reference into four possible
modes, which are summarized in Table 10. If SENSE is grounded,
the reference amplifier switch is connected to the internal resistor
divider (see Figure 38), setting VREF to 1.0 V. Connecting the
SENSE pin to VREF switches the reference amplifier output
to the SENSE pin, completing the loop and providing a 0.5 V
reference output.
If a resistor divider is connected external to the chip, as shown
in Figure 39, the switch again sets to the SENSE pin. This puts
the reference amplifier in a noninverting mode with the VREF
output defined as follows:
Table 10. Reference Configuration Summary
Selected Mode
External Reference
Internal Fixed Reference
Programmable Reference
Internal Fixed Reference
VREF
1.0µF
=
0
Figure 38. Internal Reference Configuration
5 .
VIN+A/VIN+B
VIN–A/VIN–B
×
⎛ +
⎜
⎝
0.1µF
1
SENSE
VREF
R2
R1
⎞
⎟
⎠
SELECT
SENSE Voltage
AVDD
VREF
0.2 V to VREF
AGND to 0.2 V
LOGIC
ADC
0.5V
CORE
ADC
Resulting VREF (V)
N/A
0.5
1.0
0.5
×
Rev. PrH | Page 21 of 36
⎛
⎜
⎝
1
+
R2
R1
⎞
⎟
⎠
(see Figure 39)
The input range of the ADC always equals twice the voltage at
the reference pin for either an internal or an external reference.
If the internal reference of the AD9251 is used to drive multiple
converters to improve gain matching, the loading of the reference
by the other converters must be considered. Figure 40 shows
how the internal reference voltage is affected by loading.
–0.25
–0.50
–0.75
–1.00
–1.25
1.0µF
0
0
Figure 39. Programmable Reference Configuration
0.1µF
VREF
Figure 40. VREF Accuracy vs. Load
VIN+A/VIN+B
VIN–A/VIN–B
Resulting Differential Span (V p-p)
2 × external reference
1.0
2 × VREF
2.0
0.5
SENSE
R2
R1
LOAD CURRENT (mA)
VREF = 1.0V
1.0
SELECT
LOGIC
VREF = 0.5V
ADC
1.5
0.5V
CORE
ADC
AD9251
2.0